The present invention relates generally to a beam splitter and method for generating equal optical path length beams for interferometry and other metrology applications. More particularly, the present invention is primarily directed to a beam splitter that has a monolithic structure and exhibits improved stability. The invention is particularly useful for generating parallel beams with a well defined lateral separation for a variety of applications including: surface profiling, precise angle monitoring, pencil beam interferometer and shearing interferometry.
Devices for generating parallel pencil beams are well known in the art. U.S. Pat. No. 4,498,773 to von Bieren discloses an interferometer which measures optical surfaces through the interference of two pencil beams which are reflected off an optical surface. The interferometer includes a laser for generating a laser beam which is split into two parallel beams by a beam splitter and a mirror. The two pencil beams are reflected off a second beam splitter through an alignment invariant optical device and onto the optical surface to be measured. The two pencil beams are reflected and back-trace through the alignment invariant optical device, propagate through the second beam splitter and enter an optical lens. The optical lens focuses the two beams in its back focal plane where the interference of the two pencil beams takes place. This information is then relayed through an optical spatial filter and optional micro objective into a readout section.
U.S. Pat. No. 4,884,697, which issued to the same inventors of the present invention and is commonly assigned, discloses a long-trace surface profiler for non-contact measurement of surface profile, slope error and curvature on cylindrical synchrotron radiation (SR) mirrors. The optical system is based upon the concept of a pencil-beam interferometer with an inherent large depth-of-field. The optical system uses a zero-path-difference beam splitter, which separates a laser beam into two collinear, variable-separation probe beams. A linear array detector is used to record the interference fringe in the image, and analysis of the fringe location as a function of scan position allows one to reconstruct the surface profile. The optical head is mounted on an air-bearing slide with the capability to measure long aspheric optics, typical of those encountered in SR applications. The optical system also uses a transverse outrigger beam which provides information on the relative alignment of the scan axis to the cylinder optic symmetry axis. The disclosure of U.S. Pat. No. 4,884,697 is incorporated herein by reference.
U.S. Pat. No. 5,446,710 to Gardner et al. discloses a focus error detection using an equal path length lateral shearing interferometer. As shown in FIGS. 2, 5A, and 5B, the interferometer 50 includes a beam splitter 52 and two abutting roof prisms 54 and 56. The roof prisms 56, 54 can be shifted along an axis orthogonal to the direction of light propagation and to the peak of the roof creates a lateral shear between the two output beams as shown in FIG. 5A. The roof prism may also be rotated about an axis producing a tilt between the two output beams as shown in FIG. 5B. This creates an interference pattern with a rotational orientation which is a function of the focus of the beam impinging on the optical storage medium. A pattern sensor is provided to adjust the focus of the beam in an accurate and dynamic manner.
U.S. Pat. No. 4,783,170 to Bergstrom discloses a symmetrical prismatic readout apparatus for a ring laser gyro using a platinum beam splitter. The readout apparatus includes a pair of substantially identical prism elements 12 and 14. The prism elements have sides 16, 18, 24 and 26 that are parallel and are perpendicular to sides 20 and 28 as shown in FIG. 1. The fourth side of each prism element is oriented at a 45 degree angle with respect to the parallel sides. Both the first and second prisms 12 and 14 are substantially identical in construction and are arranged so that sides 20 and 28 are collinear. A beam splitter coating 32, 34 is applied to the fourth side 22, 30 of the prism elements 12, 14. A beam splitter 36 is also imposed at the interface between the two prisms 12 and 14. The apparatus includes a reflector element 6 having an inner surface 38 that is also a beam splitter.
U.S. Pat. Nos. 5,548,403 and 5,933,236 to Sommargren disclose phase shifting interferometers. More specifically, U.S. Pat. No. 5,548,403 discloses a phase shifting interferometer that uses a spherical wavefront generated by the process of diffraction. FIG. 1 in both of the references are the same and illustrate that which is pertinent to the present invention. In particular, the interferometers include a beam splitter 22 having a pair of fixed retroreflectors 24 and 28 arranged in a similar fashion to the cube beam splitter and right angle prisms in U.S. Pat. No. 4,884,697 discussed above.
U.S. Pat. No. 4,743,188 to Tsuchiya et al. discloses a method of detecting origin of shear and measuring amount of shear in shearing interferometer systems. Referring to FIG. 1, this patent also discloses an arrangement that is similar to the disclosure in U.S. Pat. No. 4,884,697 and includes a beam splitter 18 corner-cube prisms 20, 22. The patent also includes a pair of shutters 21, 23 located between the beam splitter 18 and the corner-cube prisms 20, 22.
The problems associated with the prior art parallel beam generators include: the prior art apparatuses are not equal optical path length apparatuses thus causing instability of the measurement produced by frequency shift of the light source, or the apparatuses are subject to thermal drift and subject to additional stability problems associated with the relative mechanical vibration and slow shifting of their optical components, or both. The present invention addresses these problems through a monolithic structural design and generation of equal optical path length beams. In addition, the beam splitter in accordance with the present invention is compact, which is advantageous in certain metrology applications.
The present invention is a beam splitter for splitting an incident beam into first and second beams so that the first and second beams have a equal optical path length, fixed lateral separation, and are parallel upon exiting. The beam splitter includes a first prism, a second prism, and a film located between the prisms. The first prism is defined by a first thickness and a first perimeter which has a first major base. The second prism is defined by a second thickness and a second perimeter which has a second major base. The film is located between the first major base and the second major base for splitting the incident beam into the first and second beams.
In a preferred embodiment, the first and second perimeter are trapezoidal shaped. In this embodiment, the first perimeter also includes a first minor base, a first reflecting side, and a first nonreflecting side. The first reflecting side preferably forms a right angle with the first major base. Similarly, the second perimeter includes a second minor base, a second reflecting side, and a second nonreflecting side. The second reflecting side preferably forms a right angle with the second major base. Preferably the first nonreflecting side intersects the first major base a 45 degree angle and the second nonreflecting side intersects the second major base at a 45 degree angle. Preferably the first nonreflecting side and the second nonreflecting side are arranged to intersect at a right angle. Preferably the first major base is shorter than the second major base and the film has ratio of 1:1 between transmissivity and reflectivity after gluing.
In accordance with the present invention a method of generating beams includes providing a beam splitter and directing an incident beam at the beam splitter. The beam splitter includes trapezoidal shaped first and second prisms and a film located between the prisms. The incident beam is directed at the first nonreflecting side of the beam splitter so that the incident beam splits into first and second beams at the film. The incident beam is perpendicular to the first nonreflecting side of the prism. The first beam reflects within the first prism, passes through the film and out the second nonreflecting side. The second beam reflects within the second prism, reflects off the film to exit through the second nonreflecting side. The first and second beams are parallel and have a equal optical path length.
The second beam can be perpendicular to the nonreflecting side of said second trapezoid. The first beam can be perpendicular to nonreflecting side of said second trapezoid.
In accordance with the present invention a method of generating equal optical path length beams includes providing a beam splitter described above and providing particularly defining the configuration 2M=2A+B of the beam splitter so that the split beams have an equal optical path length, fixed lateral separation, and are parallel upon exiting.
In accordance with the present invention an interferometer for profiling the surface along one axis of an optical surface includes an incident light beam, a first beam splitter, a second beam splitter, and an image generator. The first beam splitter includes trapezoidal shaped first and second prisms and a film located between the prisms. The first beam splitter is optically oriented to receive the incident light beam at the first nonreflecting side so that the incident beam splits into first and second beams at the film. The first beam reflects within the first prism, passes through the film and out the second nonreflecting side. The second beam reflects within the second prism, reflects off the film to exit through the second nonreflecting side. With the particular configuration condition 2M=2A+B the first and second beams are parallel and have a equal optical path length while being laterally displaced from each other along the axis. The second beam splitter receives the first and second beams from the first beam splitter and passes the beams through toward the optical surface. The optical surface is oriented to reflect the first and second beams back to the second beam splitter. The second beam splitter redirects the reflected first and second beams toward the image generator which forms the profile of the optical surface. Preferably the second beam splitter is a cube and the incident light beam is a laser beam.
As a result of the present invention, a method and apparatus for generating equal optical path length beams with fixed separation and parallel upon exiting for interferometry is provided. The primary advantage of the present invention when compared with the prior art is the increase in stability due to the monolithic construction of the beam splitter combined with equal optical path length beams. In addition, the present invention provides for a compact design that is advantageous in certain metrology applications.
Using the inventor""s prior patent, U.S. Pat. No. 4,884,697, as a point of reference, the monolithic construction increases stability by eliminating the following:
1. Relative vibration of beam splitter 24 and the right angle prisms 26 and 28 (See FIG. 1 of ""697 reference);
2. Air turbulence in the air gap between the component parts;
3. Displacement problems associated with adjustment requirements; and
4. Thermal induced errors because there is now a small temperature gradient in the monolithic beam splitter.
The beam splitter of the present invention provides stability of non-reference subtraction having a slope drift as low as 0.6 micro rad rms in a thermal environment of plus or minus 0.5 degrees Celsius in 15 hours whereas the prior art had a slope drift as high as 15 micro rads rms in a plus or minus 0.1 degree Celsius environment in 10 hours.
For a better understanding of the present invention, reference is made to the following description to be taken in conjunction with the accompanying drawings and its scope will be pointed out in the appended claims.